2 inch low power for f5 newt

[Ryan101 (Ryan)]

Hey,
I'm looking at getting a low power eyepiece for my 12 inch f5 dobsonian. Something in the 40mmish sort of range.
I'm not looking for anything too expensive as it's just for general poking around and looking at the bigger nebula.
I've read on previous threads about the gso superview 2 inches and that they're not a good optical match for fast newts.


So my question is, what is a good optical match for fast newts ie: plossls, the 68 degree wide fields, kelners, etc?
I've heard that less elements is the way to go but that's all the info I can find.


Any help would be awesome. Cheers

[Saturnine (Jeff)]

Hi Ryan

The Televue Nagler and Panoptic range of eyepieces are designed to work with fast newtonians, they are not cheap though and may stretch the budget too much. The Explore Scientific range, either the 80* or 68* apparent field would be a reasonable match and not quite as pricey.
Plossels and Kellners have well corrected fields but a narrow AFOV and therefore not ideal for panoramic views of nebulae. There ae other flat field eyepieces but don't have any hands on experience with them.
I'm sure others on here will join in and give their experiences and wisdom with the wide variety of eyepieces out there. Your budget is the governing factor on how well an eyepiece will perform in your scope.

[Ryan101 (Ryan)]

Thanks Jeff



The televues are definitely put of my price range unfortunately. Maybe when the kids grow up and leave home.


I'm not looking to spend too much. Maybe a couple of hundred...max. knowing that I'm sacrificing crisp views.

You say that plossls are too small a FOV to get good wide views but are they a good optical match for higher power planetary viewing?

[AstralTraveller (David)]

Ryan,

If you can find a William Optics 28mm UWAN, they are a hidden gem. I got mine as a poor man's 31mm Nagler and I'm very happy with it. The ES have a very good reputation too. A 35mm Panoptic would also be nice but I don't know what they are worth these days.

[Startrek (Martin)]

Ryan,
I started this wonderful hobby 5 years ago with a 10” f5 push nudge dob and GSO Plössl’s 10mm , 15mm , 20mm and a 32mm Superview. It was an ideal beginner kit which provided many nights of enjoyable viewing over the first 2 years including open and globular clusters , the brighter nebula and galaxies and planetary.The Superview was my favourite eye piece at the time due to being 68deg AFV and longer eye relief, the Plössl’s were a bit of a struggle due to the narrower field of view and tight eye relief.
Fast forward to now , I’m using a Skywatcher 12” f5 Goto dob and a vast array of Televue eye pieces. My kit now is light years ahead of my beginner kit in every way, Televue eye pieces in the 12” are simply unbelievable for all types of objects.
My advice would be to buy the 32mm or 40mm Superview as they are relatively cheap and provide pretty good views then make a plan over 12 months or so and put money away each week or month to buy just one Televue eye piece.
Something like a 27mm Panoptic , 17mm Delos or a 22mm Nagler. I guarantee it will be one of your eye pieces for life in that 12” dob.

Cheers
Martin

[Ryan101 (Ryan)]

Looks like I'll just buy something cheapish then save up for a while and bite the bullet.

[mura_gadi (Steve)]

Hello,

With Martin's advice about saving for a TV, I'll add that the buying and reselling of second hand TV ep's can result in very little lose of money in the long run. If you look after the ep etc...

However a good second hand option for the impatient:
APM 30mm UFF second hand (new $335 in Oz atm) - that would allow you a decent wide view with good ER and EP.

[Ryan101 (Ryan)]

That's good to know about the televues thanks Steve.
And that eyepiece sounds like a good option. I'll keep an eye out for one.


I was talking to an ASV member last night about GSO/bintel superviews and he said the 40mm version has quite severe kidney beaning in f5 and quicker scopes, so I'm glad I didn't jump the gun on that one.

[mura_gadi (Steve)]

The other thing to remember is the ep's may have life in another scope one day. Your speed (F ratio) and telescope design may change over time.

That is the other advantage of TV's, they will always be able to go to your next scope.

******************

EPs that work well in your current F4.9 will work very well in slower scopes. Fast scopes and wide fields are a good way to hit the wallet real hard.

Look for a list of the 25 brightest/predominate objects in the southern hemisphere - re: nebulosity. Add your scope to something like astronomy tools and look at each object to see what FOV you want. Some are just too big, most you may not need 2degree FOV for either.

[Ryan101 (Ryan)]

Good point.


I do have a skywatcher refractor. I think it's 102/1000 which the other ep's may work well in.
I just have to get a new mount for it. We'll get rid of the old one then get a new one. The old one is a bresser exos 2 I think it is. But I'm no good with an eq mount.

[OzEclipse (Joe Cali)]

A 40mm eyepiece at f5 will have an 8mm exit pupil. Too big, even for young eyes. Pupil dilation maxes out around 7mm in your teens. Then, it's all downhill from there. The attached chart of pupil dilation is a population average. Individual variations are common.

Aim for something in the low 30's. I have an f5.5 dob and the Nagler 31. Great combo. Now in my late 50's, I have measured my eyes to have an approximate maximum pupil dilation of 5.5 mm. So 31mm is optimum and I don't use anything longer.

31mm Naglers work very well on all scopes so once you have saved up and purchased, will give you a lifetime of use. However not all other eyepieces work well on fast newts. If you can't afford a higher spec eyepiece, you could consider a coma corrector and then you might get reasonable results with a cheaper eyepiece in the 28-35mm range but I can't guarantee this approach will work. Perhaps someone else can comment on this approach.

[ausastronomer (John Bambury)]

On the basis that your budget is limited in the foreseeable future, It's a given that you wont be adding a paracorr any time soon. An F5 scope is borderline on whether you need a paracorr or not to correct for Edge of Field (EOF) Coma. Some people won't use an F5 scope without a paracorr, some people don't bother with one at F5. Coma increases exponentially as you go further off axis. In other words you will see a lot more Coma at the EOF in an 80 deg AFOV eyepiece than you will in a 70 deg AFOV eyepiece. On that basis I would avoid anything having an AFOV greater than about 72 degrees. In addition, as Joe mentioned a 40mm eyepiece gives an exit pupil of 8mm, which is too big. I would avoid the 40mm GSO Superview. The exit pupil is too big and to be honest its a pretty ordinary eyepiece in an F5 newtonian. You don't want to be buying equipment you know you will need to replace in the fairly short term. On the basis that your budget is limited, I would be saving up for a few weeks / months and looking for a decent 2nd hand eyepieces. Good options that will work well in your scope without a paracorr include:-

27mm TV Panoptic
APM 30mm UFF
30mm Vixen LVW (2" and very rare)
35mm TV Panoptic

You should be able to get all of those for well under $300 2nd hand. You'll just need to be patient and wait until one pops up on Astromart, Cloudy Nights or Ice Trade. The thing is you won't feel the need to replace any of them with something better in the foreseeable future.

Cheers
John B

[MichaelWB (Michael)]

I know this is a bit out of left field but how about a Meade 56mm Plossl. You would probably have to get it from O.S. It will give you just about the widest possible real fov of any eyepiece. I don't really have any idea about its suitability for your scope (I'll leave that to others with much more experience than me). I have an old Unitron 55mm Plossl that I have had for about 40 years and it works very well in my 14" f7.2 dob. I get a whole 1degree with it!!!!!

Here is a sample in the US. A lot cheaper than all the other eyepieces mentioned.

https://www.meade.com/series-4000-su...tml#additional

Good luck in your hunt for a wide field eyepiece.

Michael

[OzEclipse (Joe Cali)]

Michael,

On an f5 scope, using a 56mm eyepiece will result in an exit pupil of 11mm. Maximum pupil dilation in a teenager is 7mm, it goes downhill with age from there, see the age graph in my post above.

I don't know the age of the OP but even if Ryan is 18 yo and has a 7mm dilated pupil, using a 56mm eyepiece will result in the loss of about 60% of the telescope's light. If his eye is a 6mm dilation, the light loss increases to 70%

The specific light loss is dependent upon the maximum pupil dilation of the observer. In the attached table you can see that even if he has a full 7mm pupil dilation, he will reduce the light from his 300mm scope to that of a 190mm diameter scope. I had temporary custody of a Takahashi LE 50 some time ago and I confirmed this in my f5.5 scope. The image was very dim and unimpressive compared to my 31mm eyepiece.

Also, these eyepieces don't give you the widest possible field of view as you state. A 50mm class eyepiece will only have an apparent field of view between 40-52 degrees. The true field of view (TFOV) is at most, the same as or less than an 80 degree, 30mm class eyepiece. The TFOV with the Takahashi LE50 was roughly the same give or take 0.1 deg as my Nagler 31mm. 50mm eyepieces are usually basic plossls and are not well corrected for fast newts so you will lose the outer field.

These 50mm class eyepieces are used to gain a bigger exit pupil hence brighter image on long focal ratio telescopes, f9 and longer. SCT's, Mak's and Cassegrains are very forgiving of optical correction and these eyepieces work perfectly well.

In short, a long eyepiece is a waste of money on a short focal ratio instrument. If you're looking for something cheaper than a Nagler 31mm, the eyepieces John Bambury recommended are your best bets.

[MichaelWB (Michael)]

Hi Joe.

Your critique is freely accepted. I am more than happy to accede to others with much wider knowledge than me. The reason I said that it would have the widest fov is that on my Plossl the eyepiece id is 47mm and the field stop is over 43mm id, over 90% of possible available fov. Not the widest possible, but substantial all the same. All the other info you supply is well outside my expertise. Thanks for your enlightenment

[mura_gadi (Steve)]

Might add a very old rule of thumb for newts here,

Max mag is roughly 50x per inch of aperture, less 25%.

That's where a high end mirror shines, it gets a lot closer to the theoretical limit than some entry price mirrors. That's a fairly old rule, I would guess todays entry level mirrors are less than 25% now though.

Min Mag is the aperture in MM's /8.

That should put you around 8mm EP which as stated above is a little large.

The thing with min and max is they are normally not the best place to be. I'd stay with something around 6mm and mid to high powers from 4-2mm exit pupil.

I like a zoom and a 2" wide field, makes housekeeping around the scope a lot easier. I have a very cheap apm 30mm at 2degree fov, 5mm exit pupil and 22mm ER, the zoom is 8-24mm and 15mm-19mm ER.

[ausastronomer (John Bambury)]

Originally Posted by MichaelWB

Hi Joe.

Your critique is freely accepted. I am more than happy to accede to others with much wider knowledge than me. The reason I said that it would have the widest fov is that on my Plossl the eyepiece id is 47mm and the field stop is over 43mm id, over 90% of possible available fov. Not the widest possible, but substantial all the same. All the other info you supply is well outside my expertise. Thanks for your enlightenment

Hi Michael,

The critical difference is that your scope is F7.2; while the original posters is F5. The 55mm eyepiece is only giving an exit pupil of 7.6mm in your F7.2 scope, which is slightly oversized, but still workable without too much light loss. It's a general rule of thumb that you shouldn't use an eyepiece that gives an exit pupil > 7mm, as that is the approximate maximum eye pupil dilation of a young healthy adult. As we get older, our maximum pupil dilation reduces. Unfortunately, the same 55mm eyepiece in an F5 scope (regardless of aperture) is giving an 11mm exit pupil, which is way oversized with significant light loss. While the view may "appear" decent, the light loss would reduce the effective scope aperture from 12" to the equivalent of somewhere between an 8" and 10" scope.

Cheers
John B

[ngcles]

Hi Michael,

As others have noted, it is best to work backwards from the maximum assumed exit pupil your eyes can accommodate when fully dark adapted.

The exit pupil is the diameter of the beam of light coming out of the eyepiece when used in that 'scope. The exit pupil is approximated simply by dividing the aperture in mm by the magnification.

Assuming "about average" dilation for the healthy eye it is about 6mm. If the 'scope eyepiece combo is producing an exit pupil over 6mm your own iris is vignetting the exit pupil and not all the light from the primary mirror is actually making it into your eye.

The aperture is 300mm and the desired maximum exit pupil is 6mm, then the minimum magnification is x50 before your eye begins to vignette the light beam coming out of the eyepiece.

So the maximum eyepiece focal length you can use in 12" f/5 without the exit pupil being vignetted is therefore 30mm (1500/50). You can of course go for a longer focal length but you will be wasting light.

Lots of different views expressed here on the cheaper end of the range. You expressed interest in a "Kellner". Don't go there -- ancient design, poor aberration control and a narrower field than an Orthoscopic or Plossl.

The around 30mm generic/Taiwanese/GSO etc "wide-field" offerings ... you get what you pay for. You can expect at f/5, that at least the outermost 40% of the field will suffer from quite yucky star-images from coma and off-axis astigmatism. These eyepieces actually perform pretty well in long focal length telescopes (like a Schmidt-Cassegrainian at f/10) but not nearly so well at f/5. If you feel you can personally tolerate that long-term, fine, but there's a lot of wisdom in the idea that investing in higher quality eyepieces is a good investment because you can keep it if and use with other 'scopes if you ever decide to part with the 12". A good eyepiece is a jewel forever.

The 68 degree and 82 degree offerings from premium manufacturers incorporate more glass elements and correct for the aberrations far better. They are are expensive but you will get a far larger well-corrected field of view and the view will be commensurately much nicer.

FWIW, nearly all my eyepieces are Televue Naglers. It took me about 15 years to acquire what I call a full set (of eight), but the patience and the extra dough was worth it.

I have also done (some years ago for S&T) a test report for the 28mm UWAN David (Astraltraveller) mentioned and it is a very, very nice eyepiece. The aberration control was on par with my own 26mm T5 Nagler and it is about 60% the sticker price.

The 28mm UWAN would give you x53.5 magnification and a true field of view of just over 1.5 degrees. That's a huge FOV.

I also own a 30mm Explore Scientific 82 deg and it is excellent. It loses very little compared to the venerated 31mm T5 Nagler in aberration control but is a lot, lot cheaper.The 30mm Exploration Optics 82 deg FOV in your 'scope gives a field almost 100 arc minutes across. Expansive!

Think about a pre-loved eyepiece -- they come up from time to time on the forum here and other forums as well.As you are in Victoria, have you been to any of the observing events down there? It might provide you the opportunity to test-drive eyepieces owned by others.

By way of indicative example, here is a second-hand 28mm UWAN for sale in the US on Cloudy Nights (you will need to join to view the ad --it's free) for $170 USD. Assuming the seller is happy to sell to you in Australia (not all but most are) $170- by the time you do the USD to AUD conversion, add $25 for shipping then add GST on being landed here, you can get out of this deal for about $300 AUD and get a top quality ep.

https://www.cloudynights.com/classif...8mm-2-82d-fov/

In the end I know, your wallet/budget will have much to say. Best of luck with your choice.

Best,

L.